In this work, under atmospheric pressure and moderate temperature for various periods, e.g., 30, 45, 90, and 135 days, the air oxidation of two Central Anatolian lignites were examined. The structural changes of the lignite samples were observed by analyzing the amount of phenolic hydroxyl carboxyl, and peroxide of the samples oxidized and by comparing the IR spectra of the lignites, both original and oxidized. It was concluded that most of the phenolic structures of the lignites were oxidized into quinonoid structures during the oxidation process.